World and European champions Spain fell to a surprise defeat in their opening match of the Olympic Games men's football tournament as Japan claimed a 1-0 win with a hugely impressive performance at Hampden Park.
Yuki Otsu's first-half goal was enough for three points but in truth the margin of victory could have been much greater as 10-man Spain struggled with the pace and pressure enforced by their opponents.
The 1992 Olympic champions were already 1-0 down when they had Inigo Martinez sent-off with five minutes remaining in the first half and failed to recover from their numerical disadvantage.
Keigo Higashi, Kensuke Nagai and Hiroshi Kiyotake all went close for Japan after the break but the one goal was enough to send the Asian side top of Group D after Honduras and Morocco's 2-2 draw earlier in the day.
A second-half double from Jerry Bengtson wasn't enough to hand Honduras only their second ever win at the Olympics as 10-man Morocco held on for a draw.
The Africans enjoyed the better of the opening period and went ahead seven minutes before half-time thanks to a sweetly struck volley from Abdel Barrada.
Honduras responded excellently after the break though and were level on 55 minutes when Bengtson tapped home Maynor Figueroa's shot from close range.
Bengtson then scored again from the penalty spot, but the lead only lasted two minutes before Zakaria Labyad equalised with a scooped effort that eluded Jose Mendoza.
Zakarya Bergdich was then shown a red card for kicking out at Mario Martinez with 19 minutes remaining but Honduras couldn't take advantage of their man advantage as the points were shared.
New Chelsea signing Oscar played a starring role as Brazil were forced to fend off a brave fightback from Egypt as they won 3-2 in Group C at the Millennium Stadium.
The midfielder set up Rafael and Leandro Damiao for the first two Brazil goals, before Neymar made it 3-0 at half-time in front of a crowd of 26,812.
The South Americans took their foot off the gas after the break and were punished as Mohamed Aboutrika and Mohamed Salah pulled goals back for Egypt, but it was not enough for the Africans in this Group C clash.
Belarus marked their first appearance at a major tournament since gaining independence 21 years ago with victory over New Zealand in the other Group C clash at the City of Coventry Stadium.
An error of judgment by stand-in Kiwis keeper Michael O'Keeffe gifted Belarus midfielder Dmitry Baga the only goal on the stroke of half-time.
Luis Suarez had the last laugh on Old Trafford's boo-boys as Uruguay came from behind to beat United Arab Emirates 2-1 in their Group A opener.
When UAE skipper Ismaeil Matar gave his side a shock lead midway through the first-half, it seemed Suarez would have plenty to bother him.
But Uruguay, highly fancied for the entire tournament, kept their nerve and after Suarez had been fouled 20 yards out, Bologna midfielder Gaston Ramirez curled the South Americans level.
Suarez was also involved in Uruguay's second, as he cut inside after collecting a pass from substitute Nicolas Lodeiro, who kept going and buried a clinical shot into the far corner.
In Group B, Substitute Raul Jimenez was denied an injury-time winner by the woodwork as Mexico and South Korea played out a 0-0 draw at St James' Park.
The striker clipped the outside of the post after being put through by Tottenham's Giovani dos Santos with the fina; whistle fast-approaching.
Pierre Aubameyang denied 10-man Switzerland an opening Group B victory as Gabon claimed a hard-fought 1-1 draw.
The St Etienne striker cancelled out Admir Mehmedi's twice-taken penalty with a neat finish as the half-time whistle approached.